Interest in the application of exogenous electrical signals to the problems of tissue growth and repair arises from the discovery and study of electrical signals of endogenous origin, such as piezoelectric type signals associated with mechanical deformation of tissues, and biopotentials associated with the level of tissue viability and cellular metabolic effort. As these electrical signals contain both constant and time dependent components and location dependent components, three exogenous stimulatory signals have been studied in connection with tissue growth and repair. One such signal is the faradic type in which electrodes are placed within the tissue, in or near the location of desired stimulation. A second such signal is a capacitive type signal in which the electrodes are placed externally, on the opposite sides of the limb portion containing the tissue to be stimulated. The third such signal is the inductive type signal in which a coil or pair of coils on a common axis are placed externally so that the coil's axis passes through the limb near to the site to be stimulated.
Faradic systems have characteristic highly focal, diverging radial fields associated with them, produce high local voltage gradients, and generate a wide range of electrode products at cathode and anode. Unlike the other two signals, the use of faradic signals always superimposes the results of tissue trauma on those of electrical stimulation. Capacitive systems have dipole fields, produce relatively modest voltage gradients over large volumes of tissue, and, in the absence of constant bias, produce no electrode products, ionic and electronic current to flow between electrodes. Inductive systems, although producing local voltage gradients similar to those in capacitive systems, produce primary magnetic fields, with potential fields as a secondary effect.
One feature common to the use of the three stimulatory electrical signals is that a similar range of voltage gradients occurs in the vicinity of active osteogenesis.
Fracture healing involves several stages including: inflammation, elaboration (soft callus, hard callus for repair), and remodeling. During inflammation the biopotentials are large and negative, during elaboration (soft callus) the biopotential is falling and by the end (hard callus) the biopotential is nearly normal, and during remodeling the biopotential is normal. Thus, the resistance to the flow of current between the electrodes constitutes a means for measuring the healing process.
Electrical stimulation has been applied to the healing of defects in situations in which bony continuity is still present. Although no device tests are known to exist, acceleration of ingrowth into porous ceramic, porous metallic, and porous polymeric bodies has been demonstrated in canine models. Those persons skilled in the art desiring more information concerning the above mentioned background are referred to an article entitled "Electrical Stimulation of Hard and Soft Tissues in Animal Models" by Jonathan Black, published in Clinics in Plastic Surgery, Vol. 12, No. 2, Apr. 1985, pages 243-257.
A long term endoprosthesis is known in which a relatively large part of the surface, which when implanted makes contact with the bone of the wearer, is provided with coil type electrodes. The electrodes are disposed throughout at least 50 percent of the surface area of bone contact. The electrode spacing decreases in size in accordance with the size of the mechanical area loading on the corresponding part of the contact area. The greater the loading the smaller the spacing. A low frequency power source provides a field strength between about 1.0 to 10 mV/cm. The low frequency alternating voltage, which is produced by a receiving coil can have imposed on it a low direct voltage of a few tenths of a volt. The low voltage is produced by arranging a parallel circuit arrangement of a semi-conductor diode and a capacitor in a lead extending from a receiving coil to the electrode system. The power source is a permanent magnet whose magnetic field is cut by the coils as a hip joint carrying the device moves. Those persons skilled in the art desiring more information about this long term endoprosthesis device are referred to U. S. Pat. No. 4,214,322 issued July 29, 1980.
Thus, it is known that some specific functional link serves to transduce express electrical energies into a bone deposition, formation and cell differentiation. Cells respond and orientate to direct current field patterns under stimulation of cathode placement. In addition a small thin invasive electrical stimulatior device is described in U.S. Pat. No. 4,602,638 issued July 29, 1986 that can be easily implanted and which is operative to both monitor the healing process and to adjust the electrical stimulation in response thereto.
In this device, a pair of insulated leads connect the cathode and anode to an implantable power pack which includes a source of electric power and an insulating encasement. The cathode may be laminal in construction and includes a plurality of spaced-apart frequency modulation (FM) channel monitors embedded within an insulation layer adjacent the conductive electrode forming sheet of material. The FM channel monitors are electrically connected to an FM telemetry component within the power pack for transmitting signals indicative of the electrical characteristics of deposited bone callus, cartilage and soft tissue and therefor, the healing response of a fracture to electrical stimulation. A frequency modulated current regulator may be added to the power pack for adjusting the amperage in response to FM control signals from a remote transmitter. Adjustment of the current flow may be automatically accomplished by providing an implantable computer assisted device responsive to external programming communicated to it by a remote FM transmitter.
Disadvantages of such known systems include the need: to enhance the formation of bone to an implant interface; to control the location of the bone growth to eliminate separation of the bene from the endoprosthesis device resulting from stresses such as those generated by normal body movement; and to provide a less sophisticated means for monitoring the healing process results.